What are indexes in databases, why are they needed, and what is their internal structure?

Database indexes are special data structures that speed up searching and sorting records based on specific table fields. Without indexes, searching requires a full scan of all rows, which is slow with large data volumes.

Why indexes are needed:

• Accelerate SELECT operations with WHERE, JOIN, ORDER BY conditions.
• Improve performance during data retrieval.

The internal structure of indexes depends on the database type and index type, but the most common are:

1. B-trees (B-tree) — balanced trees where each node contains keys and references to child nodes. They allow quick range and individual key searches.

2. Hash indexes — use hash functions for fast access by exact key match, but are not suitable for range queries.

3. Bitmap indexes — effective for columns with a small number of unique values.

Example of a B-tree structure:

• Root, internal nodes, and leaves.
• Leaves store references to table records.
• Search starts from the root, comparing keys and moving along branches.

Indexes require additional space and slow down insert/update operations, so they should be used thoughtfully.